Potential for Cure Time Reduction in Rubber Injection Molding and its Analytical Prediction

In rubber injection moulding reduction of cure time is a long-term topic. Due to high part thicknesses in the
range of several centimetres the cure time in most cases reaches amounts to several minutes. Therefore not
only the chosen curing temperature should be reached very fast but also a homogeneous crosslinking degree
must be achieved over the whole part volume. Reduction of cure time by means of shear heating is
state-of-the-art. Devices at the injection molding machine which generates coupled shear and elongational
flow are in practical use but up to now available software tools for injection molding simulation neglect the
effect of elongational heating.
In [1] as engineering approach a new viscous model for the prediction of temperature changes in rubber
compounds flowing through conical dies and runner segments was deduced and experimentally validated. The
analytical calculation program developed takes into account shear as well as elongational flow and allows a
precalculation of the temperature increase with accuracy of 4 %. As material data input shear and elongational
viscosity has to be measured. For SBR and NBR compounds it was shown that more than 80 % of the
temperature increase measured is caused by elongational flow.
Systematic use of elongational flow generating dies in the injection mould (Figure 1) allows considerable
dissipative heating [1]. For highly viscoelastic SBR and NBR compounds a temperature increase by 45 °C to
scarcely 70 °C could be reached (Figure 2). Conditions precedent are conical dies with a half die angle of
higher than 45° and high injection speed. This type of dissipative heating preferably appears around the centre
line of the flow, and in combination with shear heating near to the wall a more homogeneous temperature
profile at a higher temperature level is possible.
Figure 1: Injection mould with elongational flow Figure 2: Measured and calculated bulk for
generating dies tested temperature increase for NBR
The potential for reduction of cure time can easily be estimated in combination with a self-developed
calculator for cure time. For rubber parts with thicknesses higher than 4 mm the potential ranges from 5 % up
to more than 30 % depending on mould temperature and wall thickness. By far the highest contribution to
heating is caused by elongation as the polymer mass flows into a die. This was found to account for approx. 80
percent of heat introduced into a NBR compound, compared to just 10 percent caused by shear at the die
walls.It is recommended to implement elongational flow generating dies in the machine nozzle area or in case
of cold runner systems directly at the gate. The level of reachable dissipative energy depends on the die
geometry of the maximum injection pressure available.
This project was financially supported the Austrian Research Funding Agency FFG and the company partners
Maplan GmbH, Semperit technische Produkte GmbH and Erwin Mach Gummitechnik GmbH.